There are known in the art machines for handling sheet materials and comprising means for the displacement of the nipper bars fastening the sheet materials, a movable lower platen and a fixed upper platen, the sheet materials being fed between them, and a movable platen control mechanism.
There are diverse control mechanisms for said movable platen.
In one of the known control mechanisms described in Swiss Patent No. 245082operation of the movable platen is performed by means of a crankshaft which is driven at a constant speed and acts through connecting rods upon an ensemble of cranked levers.
According to another mechanism described in Spanish Patent 521461 , operation of the lower platen is performed by means of direct drive cams upon an ensemble of cranked levers. This device uses cams mounted on a shaft placed between the several groups of cranked levers. In this embodiment, the movement generated by the cams is transmitted to the groups of cranked levers by means of a rolling member provided in said groups of cranked levers and which follows the cam outlines, providing for adjustment of the conditions of stroke, speed and acceleration of the lower platen movement.
This device has, however, the drawback that it must be provided with mechanisms ensuring always the contact of the rolling member with the cam, which increases its complexity and makes assembly of the device difficult.
A specific prior art apparatus, disclosed in Spanish Patent Application 88 00844 of the applicant in the instant application, comprises a movable platen control mechanism in which the central joints of the crank levers are horizontally moved by a crankshaft driven by a revolving shaft receiving driving movement from a worm-and-wheel mechanism and rocking drive means.
This rocking drive means comprises a rack segment rotating on an axle coupled to a wheel at a given distance from the rotating wheel axle and a sprocket coupled to the wheel axle. The rack segment has a pin forced to run along a groove in such a way that every turn of the wheel causes a reciprocatory motion in the rack segment which turns into a rocking motion in the sprocket and in the crankshaft.
Owing to this rocking drive means, when the worm mechanism revolves at a constant speed the crankshaft does so at a variable speed which is slower at the lower position of the platen and faster at the upper position adjacent to the fixed upper platen, thus facilitating the synchronized passing of the nipper bars between the platens.
In this way, the movement of the lower platen adapts to the movement of the sheet feeder element at every working cycle of the press. Said sheet feeder element consists of nipper carrying bars. In these bars, the pressure surface for the nippers is located, to improve the aerodynamic behavior of the bars, between the upper and the lower planes of the bars.